An antenna is a unit for transmitting and/or receiving electromagnetic radiation. It is used in numerous communication systems that require one or more of these radiating units.
When several antennas are associated for a single common function, the set of these antennas is known as a network of antennas. A network of antennas provides certain advantages in relation to one antenna, such as providing greater directivity, the equivalent radiating aperture of a network being greater than that of a single antenna within this network.
Helical antennas, i.e. antennas formed by a conductive wire whose path describes a helix, are used in many systems such as mobile phones, sources for the focal points of a concentrating system or the major antenna networks.
FIG. 1a shows the projection of a helical antenna 100 onto a plane comprising the longitudinal axis 104 of the antenna and FIG. 1b shows the projection view of the antenna 100 on a plane perpendicular to the longitudinal axis 104 of the antenna.
FIG. 1b enables the interior 105 of the helix to be defined as the volume generated by the cylinder of an axis and radius equal to the axis and radius of the helix, the exterior 107 of the helix being the volume surrounding the interior 105.
A helical antenna 100 comprises one or more conductive wire segments 102 of a helical or spiral form. Each segment 102 can be defined by its length 108 according to the longitudinal axis 104, by the number of turns 110, by the cross-section 112 of the wire, by the pitch 114 of the helix, by the angle of inclination 116 of the helix (angle between a tangent to the helix and a plane perpendicular to the axis 104 of the antenna), by the radius 106 of the helix and by the nature of the conductive material forming the wire.
Moreover, it must be noted that these parameters can be variable or constant along the axis 104 of the helix, and particularly along a segment 102.
Such a helical antenna 100 has several operating modes that depend on various parameters such as its radius 106, its angle 116 of inclination of the turns 110 and its pitch 114.
According to a first polarisation mode, a helix can be polarised along its longitudinal axis 104 (linear polarisation) in a mode called normal, this linear polarisation being used particularly in mobile phones.
According to a second mode, a helix can also be polarised in a circular manner (circular polarisation in a mode known as axial) around the helix axis when the helix radius is in the order of the wavelength of the waves to be transmitted or received, this circular polarisation being generally used, for example, for the helical antennas present at the focal points of parabolic antennas used to receive and transmit electromagnetic waves coming from satellites.
In this latter case, the quality of the circular polarisation depends on the number of turns 110, whereas the directivity depends on the length 108 of the antenna.
It is known that a helical antenna is manufactured according to several distinct methods, these methods being described hereafter:
A first manufacturing technique consists in conferring a helical profile on a conductive wire by forming, in a manner analogous to the manufacture of a spring.
A second manufacturing technique consists in winding a conductive wire around an insulating element such as a plastic tube or a foam block, the insulating element being used to support the helix.
The insulating element can subsequently be left or removed if the strength of the helical antenna is great enough to retain its shape without the support.
A third technique consists in printing one or more conductive lines diagonally on a substrate, for example, a sheet of an insulating element that is folded onto itself, thus forming a helix whose angle of inclination is the angle of the diagonal on the substrate.
Moreover, when antennas are manufactured individually, it may be necessary to group these antennas into a network, in which case these antennas must be linked together by a rigid conductive element and a driver circuit.
These methods for manufacturing antennas have various drawbacks. Hence, the forming technique requires sufficient rigidity from the conductive element whereas the manufacturing methods according to the second and third techniques, i.e. by winding around a support or by folding a substrate, are relatively complex and costly to implement, particularly owing to the required deformation of the conductive wire (for winding) or the substrate (for folding).
Finally, these known methods only apply to the manufacture of individual antenna of such a sort that grouping several antennas into a network requires other specific operations, thus increasing the cost of manufacturing a network.
During these operations specific to grouping antennas into a network, it must be noted that each antenna must be connected to the entire network with a mechanical rigidity sufficient to retain, over a long period, the polarisation characteristics of the antenna network in its entirety, which is particularly complex and costly.
Now, it is known that, in a network, the antennas must keep their directions with respect to each other and with respect to the driver circuit, so that the driver circuit can maintain its performances.